1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and particularly to a method for manufacturing a flexible semiconductor device which can be bent.
2. Description of the Related Art
In recent years, a semiconductor element provided over a rigid substrate such as a glass substrate has been actively developed for use in a display and a photoelectric conversion element such as an LCD, an organic EL display, a photo sensor, and a solar cell. On the other hand, as for an element using a Si wafer, an IC chip has been miniaturized and thinned for use in a cellular phone and the like. In addition, a semiconductor device which transmits and receives data without contact (also referred to as an RFID (Radio Frequency Identification) tag, an ID tag, an IC tag, an IC chip, an RF (Radio Frequency) tag, a wireless tag, an electronic tag, or a wireless chip) has been actively developed. In any case of using a rigid substrate such as a glass substrate, a Si substrate, or the like for manufacturing such a semiconductor device, a reduction in thickness of a substrate is required along with needs for miniaturization and thinning.
In addition, recently, a flexible device has been required for an RFID tag embedded in paper, a display which can be wound around a pen, a profile sensor for a three-dimensional shape or a color sensor, a hand roll PC, clothes of which design is changed by changing the color, or the like. Therefore, a reduction in thickness holds an important key.
In the case of forming a semiconductor element using a pre-thinned substrate to manufacture a thin semiconductor device, warpage of the substrate due to stress, difficulty in handling, and misalignment in lithography, a printing step, and the like become problems. Consequently, a method for thinning a substrate after forming a semiconductor element over the substrate is generally used.
As for a reduction in substrate thickness by grinding or polishing, conventionally, a thinner film is formed while improving substrate planarity using an abrasive grain as a polishing step after thinning a substrate using a grindstone as a grinding step. An abrasive grain having lower Vickers hardness than that of a substrate to be polished tends to be used as a device for improving planarity. For example, cerium oxide (CeO2) for a glass substrate, silicon oxide (SiO2) for a silicon wafer, or the like, which has lower Vickers hardness than that of the substrate, makes it possible to selectively polish only a portion in close contact with an object by chemical reaction (for example, see Reference 1: Japanese Patent Laid-Open No. 2004-282050).
In addition, there is a technique for removing a glass substrate by wet etching using chemical reaction (for example, see Reference 2: Japanese Patent Laid-Open No. 2002-87844).
However, when a substrate is thinned or removed after providing a semiconductor element over the substrate, it is concerned that an impurity element, moisture, or the like from outside easily enters the semiconductor element and adversely affects the semiconductor element.